The present invention generally relates to an image reading apparatus, and more particularly to an image reading apparatus having a photoelectric conversion element in which a plurality of cells are arranged in a line.
A photoelectric conversion element such as a CCD sensor is used for an input device of a copy machine, a facsimile, an image filing apparatus or the like, which input device reads an image formed on a document. In the photoelectric conversion element, a plurality of cells are arranged in a line so that an image can be read at a resolution in a range between a value of a few dots per millimeter and a value of several tens of dots per millimeter. Electric charges integrated in respective cells of the photoelectric conversion element are shifted to an analog transmission path and then the electric charges are successively transmitted in the analog transmission path so that a main scanning is carried out in a direction of the arrangement of the cells, and the photoelectric conversion element moves in a direction perpendicular to the direction of the arrangement of the cells so that a sub scanning is carried out. Due to the main scanning and the sub scanning by the photoelectric conversion element with respect to the document, the image formed on the document can be read. An image reading apparatus having the photoelectric conversion element as described above is, for example, disclosed in Japan Patent Laid Open Publication No. 62-235872.
In the photoelectric conversion element, each cell outputs an image signal which is the sum of a photoelectric conversion signal corresponding to an exposure of the document and a dark signal corresponding to a dark current which is generated in each cell independently from the exposure thereof. Thus, to accurately obtain image signals corresponding to information formed on the document, it is necessary to correct the image signal obtained from each cell of the photoelectric conversion element so that the dark signal is removed from the image signal obtained from each cell.
Conventionally, the following apparatuses each having a function for correcting the signal obtained from each cell of the photoelectrical conversion element have been proposed.
In a first image reading apparatus, several cells positioned at an end portion of the photoelectric conversion element are optically shielded. Each of the cells optically shielded outputs only the dark signal. When the document on which an image is formed is read by the photoelectric conversion element, an average, for example, of the dark signals output from the cells optically shielded is removed from the image signal output from each of the cells arranged in an effective area for obtaining information about the document. However, in the first image reading apparatus, as the dark currents generated in the cells are different from each other, it is difficult to accurately remove the dark signal from the image signal output from each cell arranged in the effective area.
In a second image reading apparatus, a dark signal which is output from each cell under a condition where there is no exposure is stored in a memory. Then, when the document on which an image is formed is read by the photoelectric conversion element, the dark signal stored in the memory for each cell is removed from the image signal output from each corresponding cell of the photoelectric conversion element. In the second image reading apparatus, when the image formed on the document is read, at an initial time the dark signal can be accurately removed from the signal output from each cell. However, as the dark signal for each cell varies with the passage of time, it gradually becomes difficult to accurately remove the dark signal from the image signal output from each cell. In general, the dark signal for each cell greatly varies in accordance with the temperature of the photoelectric conversion element. In a CCD image sensor, for example, when the temperature of the photoelectric conversion element increases by 8.degree. C., the level of the dark signal for each cell is doubled. That is, in a case where a reading time of a few seconds is required for reading one document on which an image is formed, the dark signal for each cell varies with the passage of time while the image is being read.